Tri Power Idle Circuit Changes For Bigger Cam?

[32witha409]

I have been driving my 60 348 tri-power for a while. The idle and off idle performance is poor and acts lean. Idle screws do little and are out 2-1/2-3 turns. Total timing 34*. initial 14-15*, idle manifold vacuum hot is about 8"hg, and swings +/- with the speed variation. Yes the throttle plates are open quite a bit so likely close to the idle transition hole effectiveness.
I looked for and tested for(carb cleaner around the bases) air leaks) nothing found.
I pulled the center carb and everything looks OK. All fuel restrictions are basically OEM, mains are 60's.
My initial reaction is to up the idle fuel tube orifice .003-.004 and up the upper restriction about the same (passes fuel and air emulsion). Before I do that I wanted your opinions, and what you have had to do after adding a larger duration cam. FYSA, this was needed on my 409/425 after installing a bigger bump stick.
TIA Robert

 

[La Hot Rods]

Are the outside carburetors closed completely?

 

[32witha409]

Are the outside carburetors closed completely?

Thanks for the reply. That was the first thing I checked, both running, and after the center carb was removed, they are fully shut. I may pull them and make sure the butterfly's are sealing if nothing else helps.

 

[La Hot Rods]

With it running, put your hand over the top of the outside carburetors, you should not feel any vacuum.

 

[32witha409]

Not a bit of flow is apparent by hand.

 

[La Hot Rods]

How do you have your distributor timing curve set up?

 

[32witha409]

Nothing special, Vac advance not hooked up, centrifugal all in by 3000.. Interesting cuz the vac advance may help bring in more idle timing. That could help and allow a smaller idle throttle plate opening. Will look into that later.

 

[Fathead Racing]

With the vacuum advance connected the idle will come up allowing you to close the curb idle screw and get you out of transition , this in turn this will allow you to adjust the air idle bleeds to lean. Also, with the idle vacuum at 8 inches the power valve could be open in the center carb resulting in a rich condition. You may have to shim the spring.

 

[scott hall]

We had to up the whole size in ours to get the idle to be correct. Can’t remenber how big we went. It was enough that the idle screws are out 1 1/2 and after about a minute idles without the choke.

 

[32witha409]

With the vacuum advance connected the idle will come up allowing you to close the curb idle screw and get you out of transition , this in turn this will allow you to adjust the air idle bleeds to lean. Also, with the idle vacuum at 8 inches the power valve could be open in the center carb resulting in a rich condition. You may have to shim the spring.

Funny because the power plunger was frozen down (rich), I freed it up. Looks like I need lower spring force not higher. Shimming would go to power earlier?

 

[32witha409]

I'll measure the idle tube size once I get my gauge pins, it is under .032 and a 60 drill is .04. The upper restriction is .040, mains are .060"

 

[Fathead Racing]

Funny because the power plunger was frozen down (rich), I freed it up. Looks like I need lower spring force not higher. Shimming would go to power earlier?

For some reason me thinks the opposite is what you think. It's been a long time but I think the power valve in the 2G carb doesn't work the way the carters work. I'll have to think on it and let you know. But think of this, when idleing you would see a high vacuum condition and it would take higher spring pressure to keep the valve closed, lower vacuum pressure valve opens.

 

[32witha409]

You want the plunger to stay up in vacuum that would be lower in a bigger cam engine. The vacuum works against the spring just like a AFB. In my case 8-10 inhg at idle will cause the PV spring to over come the vacuum and the plunger will activate the PV

 

[Fathead Racing]

I don't think that's the way it works in the Rochester.

 

[32witha409]

From the manual.
To obtain the proper mixture required when more power is desired or sustained high speed driving is to be maintained, the Rochester 2-bore carburetor uses a vacuum operated power system. The spring loaded power piston, located in a cavity in the air horn, directly above the power valve, is held in the up position by engine manifold vacuum at all times during engine operation. During idle and cruising ranges, the relatively high engine vacuum holds the power piston stem. During this period, the power valve remains closed. Increases in engine load lower the manifold vacuum. When it has dropped sufficiently, the power piston spring overcomes the upward vacuum pull and the power piston moves downward, opening the power valve to allow additional fuel to flow through calibrated restrictions and on into the main metering jets to give the proper mixtures required for power operation.

 

[Fathead Racing]

I deleted that post after reading the manual post that you have. My memory of how I treated this problem has been lost. But I did have that problem and overcame it.

 

[La Hot Rods]

The power piston is in the top, spring pushes it down when vacuum drops.

 

[Don Jacks]

What in thunder cam is in that engine to cause that low of an engine vacuum reading at an idle?

 

[32witha409]

What in thunder cam is in that engine to cause that low of an engine vacuum reading at an idle?

Good question, I did not build the engine. The builder has passed so there is a dead end, no pun intended. I think most of the issue is the lean condition. Once off idle it is up to 12-15. I'll solve this problem soon, likely next weekend though. Have to wait for parts and work.

 

[Don Jacks]

Hook the vac advance back up,reset idle speed,note vacuum as you adjust idle mixture.You may be right about needing to adjust the idle/air circuts but these carbs were set up to use the vac.advance at idle.You could go too rich.Here's where an air/fuel gauge would come in really handy.